In a call under a noisy environment using a communication terminal device, such as a portable telephone set (known also as “MS (mobile station)” or “mobile phone”) which is connected to the other party's telephone terminal through a switching center, a telephone network or the like connected, through a wireless line, to a base station constituting a mobile communication system, in some cases, the contents of the other party's speech are hindered by surrounding noise and cannot be heard without difficulties. The realization of a portable telephone set, which can understand the contents of the other party's speech even under a noisy environment and can have a conversation with the other party even under a difficult-to-hear environment, can increase the use range and convenience.
For example, Japanese Patent Laid-Open No. 168552/1999 discloses a portable telephone set provided with voice recognition means for converting voice signals to character data. Specifically, the portable telephone set has a voice memo function, i.e., the function of recording the contents of conversation and the like during a call, and comprises voice recognition means for converting voice data to character data. This portable telephone set is constructed so that data size necessary for a voice memo per call is significantly reduced by writing converted character data in a memory. In the portable telephone set described in Japanese Patent Laid-Open No. 168552/1999, in order to effectively utilize the memory for a voice memo, voice data are converted to character data which are then stored in the memory. That is, this portable telephone set lacks in means for permitting the user to immediately output the other party's speech.